This disclosure is based on application No. 00-0038993 filed in Japan on Feb. 17, 2000, the entire contents of which are hereby incorporated by reference.
1) Field of the Invention
The present invention relates to piezoelectric devices and, more particularly, to a roll-type piezoelectric conversion element formed as a tube by rolling up at least one piezoelectric sheet element and to methods of making the same. The roll-type piezoelectric conversion element may be used, for example, as an actuator.
2) Brief Description of Related Art
Actuators using piezoelectric conversion elements are used to drive and position driven parts in cameras, measuring devices, and other precision machinery because they have high conversion efficiency for converting an applied electrical energy to a drive force, are compact and light weight, and are capable of generating large drive forces. Further, the drive force is readily controllable.
A drive source piezoelectric conversion element used as an actuator may be constructed by laminating a plurality of single piezoelectric elements. This arrangement increases as much as possible the displacement generated in the thickness direction of a single piezoelectric element.
However, piezoelectric conversion elements (also referred to as piezoelectric conversion devices) constructed by laminating a plurality of individual piezoelectric elements are expensive because they are typically manufactured through complex operations including a process to apply an electrode to the surface of the individual piezoelectric elements, a process for lamination and adhesion of the piezoelectric elements, and a process for connecting the wiring of the electrodes of the various layers.
For this reason, other piezoelectric conversion elements have been proposed. One type is a conventional roll-type piezoelectric conversion element formed by laminating two thin piezoelectric sheet elements having electrodes formed on their surfaces so as to form a laminate member which is then rolled in a hollow tube shape (hereinafter referred to as xe2x80x9ctwo-layer typexe2x80x9d). Another type is a conventional roll-type piezoelectric conversion element formed by folding a single thin piezoelectric sheet element having electrodes formed on the front surface and back surface so as to form a lamination which is then rolled into a hollow tube shape (hereinafter referred to as xe2x80x9csingle-layer typexe2x80x9d).
The conventional two-layer type piezoelectric conversion element is provided with an electrode on the entire front surface of each of the two layers of individual piezoelectric sheet elements. The two layers are laminated and rolled into a hollow tube shape. In order to apply a voltage to the electrodes, the two electrodes are exposed on the side surface of the piezoelectric sheet elements when forming the hollow tube shape by staggering the position of the rolled ends of the two individual piezoelectric sheet elements. Such a configuration is illustrated in FIG. 10, which shows a perspective view of an example of a conventional two-layer piezoelectric conversion element 100. FIGS. 11(a) and 11(b) illustrate the electrode surfaces and the lamination state of this element.
A conventional single-layer type piezoelectric conversion element, such as illustrated in FIGS. 12 and 13, is formed by folding approximately in half a single thin piezoelectric sheet element having electrodes formed on the front surface and the back surface, thereby creating a lamination. The folded part is shifted slightly to the right or left from the center part of the single layer element, such that both ends of the folded unit piezoelectric sheet element are shifted to expose the two electrodes on the side surface of the piezoelectric conversion element when formed in a hollow tube shape, thereby allowing leads to be readily connected to the electrodes.
The process for manufacturing the conventional two-layer type piezoelectric conversion element 100 is described below. First, a thin sheet-like material called a xe2x80x9cgreen sheetxe2x80x9d formed of piezoelectric ceramic material is cut to suitable dimensions to provide a first piezoelectric sheet element 101 and a second piezoelectric sheet element 102, as shown in FIG. 11(a). The length of the second piezoelectric sheet element 102 in the rolling direction is longer by a measure d than the first piezoelectric sheet element 101.
A first electrode 103 is formed on the surface of the first piezoelectric sheet element 101, and the back surface is designated a non-electrode surface. A second electrode 104 is formed on the surface of the second piezoelectric sheet element 102, and the back surface is designated a non-electrode surface (refer to FIG. 11a).
Then, the first piezoelectric sheet element 101 is arranged adjacent to the second piezoelectric sheet element 102 such that the non-electrode surface of the first piezoelectric sheet element 101 confronts the electrode surface of the second piezoelectric sheet element 102, forming a laminate body as shown in FIG. 11(b). This laminate body is rolled using a rolling shaft formed of cellulose or the like so as to form the tube-like shape shown in FIG. 10. Thereafter, the tube is calcined at a specific temperature, and subjected to a polarization process to complete the piezoelectric conversion element. Selecting the appropriate calcination temperature and polarizing conditions depends upon the particular piezoelectric material utilized and is within the purview of one of ordinary skill in the art. The rolling shaft is incinerated by the calcination process, leaving the interior of the tube hollow.
As shown in FIGS. 10 and 11(b), when the length of the first piezoelectric sheet element 101 is shorter than the length of the second piezoelectric sheet element 102 in the rolling direction, the ends of the first electrode 103 and the second electrode 104 can be staggered, and a lead 103a and a lead 104a can be easily connected to the respective electrode.
A conventional single-layer type piezoelectric conversion element is similar to the conventional two-layer type piezoelectric conversion element 100 described above. FIGS. 12 and 13 illustrate a conventional single-layer, roll-type piezoelectric conversion element 200 formed by folding in half a single-layer piezoelectric sheet element 201, which is then rolled up to form a hollow tube shape. FIG. 12 is a cross section view, and FIG. 13 is a perspective view. The construction of the conventional single-layer type piezoelectric conversion element 200 will now be described in more detail.
A piezoelectric sheet element 201 formed of a material called a xe2x80x9cgreen sheetxe2x80x9d having a thin sheet shape and formed of a piezoelectric ceramic material is cut to a suitable dimension. A first electrode 203 is formed on the front surface of the piezoelectric sheet element 201, and a second electrode 204 is formed on the back surface thereof. Then, the piezoelectric sheet element 201 is folded in half from a position 205 at the approximate center, forming a lamination as shown in FIGS. 12 and 13. This laminate body is rolled up in a tube shape in the same way as the previously described two-layer piezoelectric conversion element, calcinated at a specified temperature, and subjected to a polarization process to complete the piezoelectric conversion element.
A conventional tube-shaped two-layer type or single-layer type piezoelectric conversion element (100 or 200) of the aforesaid construction may suffer from short circuiting problems. As described above, a conventional piezoelectric conversion element (100 or 200) is formed by rolling up a laminate body into a tube shape, such that the piezoelectric conversion element (100 or 200) has many overlapping layers. This tube-shaped conversion element (100 or 200) is then calcinated at a specified temperature. During the calcination process, a difference in contraction occurs between the electrode and green sheet layers of the piezoelectric laminate body, causing peeling between layers of the laminate body due to this difference in contraction. This contraction often results in defects such as cracks in the piezoelectric sheet element, which can cause short circuiting between electrodes when these defects cross the layers of the electrically insulating piezoelectric sheet element.
When the conditions of short circuiting between electrodes of conventional piezoelectric conversion elements were investigated, it was found that a large contraction of the piezoelectric sheet element (green sheet) occurred due to calcination, and that the remaining electrode at the end of the piezoelectric sheet element (green sheet) curved over the piezoelectric sheet element such that this remaining electrode contacted the electrode of the under layer, thereby creating a short circuit between electrodes. This situation is illustrated in FIG. 4(b) for a conventional piezoelectric conversion element wherein the end of electrode 12x curves over the end of the piezoelectric sheet element 11x after calcination, thereby contacting electrode 16x. Such short circuiting is detrimental to the operation of a piezoelectric device.
Accordingly, an object of the present invention is to provide a piezoelectric conversion element having excellent yield that does not suffer from short circuits between electrodes created by calcination of the piezoelectric sheet element. It is also an object of the present invention to provide a method of fabrication of a piezoelectric conversion element that has excellent yield and that does not suffer from such short circuiting.
According to a first embodiment of the invention there is provided a piezoelectric conversion element having a tube shape comprising a piezoelectric sheet element and an electrode formed on a surface of the piezoelectric sheet element, the piezoelectric sheet element and the electrode being configured in the form of a rolled-up laminate. The surface of the piezoelectric sheet element includes a non-electrode surface portion without electrode material disposed thereon, the non-electrode surface portion being located at an end of the piezoelectric sheet element in a direction intersecting a tube-axis direction. The piezoelectric sheet element may comprise a ceramic piezoelectric material, and the ceramic material may be a PZT ceramic. In addition, the non-electrode surface portion may have a predetermined width approximately equal to a thickness of the piezoelectric sheet element. The thickness of the piezoelectric sheet element may be a thickness prior to a calcination process.
According to another embodiment of the invention, there is provided a piezoelectric conversion element having a tube shape comprising a first piezoelectric sheet element having an electroded surface and an opposing non-electroded surface disposed opposite the electroded surface of the first piezoelectric sheet element. The piezoelectric conversion element further comprises a second piezoelectric sheet element having an electroded surface and an opposing non-electroded surface disposed opposite the electroded surface of the second piezoelectric sheet element. The first piezoelectric sheet element and the second piezoelectric sheet element are configured in the form of a rolled-up laminate such that the non-electroded surface of the first piezoelectric sheet element and the electroded surface of the second piezoelectric sheet element confront each other. The electroded surface of the first piezoelectric sheet element includes a first non-electrode surface portion without electrode material disposed thereon, and the electroded surface of the second piezoelectric sheet element may include a second non-electrode surface portion without electrode material disposed thereon. The first non-electrode surface portion may be located at an end of the first piezoelectric sheet element in a direction intersecting the tube-axis direction, and the second non-electrode surface portion may be located at an end of the second piezoelectric sheet element in a direction intersecting the tube-axis direction. The electroded surface of the first piezoelectric sheet element may be arranged on a radially-outward-facing side of the first piezoelectric sheet element. In addition, the electroded surface of the first piezoelectric sheet element may include a third non-electrode surface portion without electrode material disposed thereon, and the electroded surface of the second piezoelectric sheet element may include a fourth non-electrode surface portion without electrode material disposed thereon, such that the third non-electrode surface portion is located at another end of the first piezoelectric sheet element in a direction intersecting the tube-shape axis direction and such that the fourth non-electrode surface portion is located at another end of the second piezoelectric sheet element in a direction intersecting the tube-axis direction. The first non-electrode surface portion may have a predetermined width approximately equal to a thickness of the first piezoelectric sheet element, and the second non-electrode surface portion may have a predetermined width approximately equal to a thickness of the second piezoelectric sheet element, wherein the thicknesses referred to may be thicknesses prior to a calcination process.
According to another embodiment of the present invention, there is provided piezoelectric conversion element having a tube shape, comprising a piezoelectric sheet element having a first electrode formed on a first surface thereof and having a second electrode formed on an opposing second surface thereof, the piezoelectric sheet element having a fold parallel to a tube-axis direction, the piezoelectric sheet element being configured in the form of a rolled-up laminate. The first surface of the piezoelectric sheet element includes a first non-electrode surface portion without electrode material disposed thereon, the first non-electrode surface portion being located at a first end of the piezoelectric sheet element in the direction intersecting the tube-axis direction. In addition, one of the first and second surfaces may include a second non-electrode surface portion without electrode material disposed thereon. The second non-electrode surface portion may be located on the first surface at a second end of the piezoelectric sheet element, the piezoelectric sheet element being configured such that the fold causes the second surface to confront itself. Alternatively, the second non-electrode surface portion may be located on the second surface at one of the first end and a second end of the piezoelectric sheet element, the piezoelectric sheet element being configured such that the fold causes the second surface to confront itself. The first and second non-electrode surface portions may have a predetermined width approximately equal to a thickness of the piezoelectric sheet element, and the thickness referred to may be the thickness prior to a calcination process. In addition, at least one of the first and second surfaces of the piezoelectric sheet element may be provided with a non-electrode surface portion without electrode material disposed thereon in a region encompassing the fold. Moreover, the fold may be displaced a distance from the center of the piezoelectric sheet element such that ends of the piezoelectric sheet element are displaced relative to one another in the direction intersecting the tube-axis direction.
According to another embodiment of the present invention, there is provided a method of fabricating a piezoelectric conversion element, comprising arranging a first piezoelectric sheet element adjacent to a second piezoelectric sheet element, the first piezoelectric sheet element having an electroded surface that includes a first electrode and having an opposing non-electroded surface, the second piezoelectric sheet element having an electroded surface that includes a second electrode and having an opposing non-electroded surface. The non-electroded surface of the first piezoelectric sheet element may confront the electroded surface of the second piezoelectric sheet element. The method further comprises rolling the first and second piezoelectric sheet elements around a shaft, the first and second piezoelectric sheet elements thereby forming a tube member having overlapping layers of the piezoelectric sheet elements with a tube-axis direction parallel to the shaft. The method further comprises calcinating the tube member at a temperature sufficient to bind the layers of the tube member together, the shaft being incinerated during the calcinating, wherein the electroded surface of the first piezoelectric sheet element includes a first non-electrode surface portion without electrode material disposed thereon, and wherein the electroded surface of the second piezoelectric sheet element may include a second non-electrode surface portion without electrode material disposed thereon. The first non-electrode surface portion may be located at an end of the first piezoelectric sheet element in a direction intersecting the tube-axis direction, and the second non-electrode surface portion may be located at an end of the second piezoelectric sheet element in a direction intersecting the tube-axis direction. The first non-electrode surface portion may have a predetermined width approximately equal to a thickness of the first sheet of piezoelectric material, and the thickness referred to may be the thickness prior to calcinating. The second non-electrode surface portion may have a predetermined width approximately equal to a thickness of the second sheet of piezoelectric material, and the thickness referred to may be the thickness prior to calcinating. In addition, the electroded surface of the first piezoelectric sheet element may include a third non-electrode surface portion without electrode material disposed thereon, and the electroded surface of the second piezoelectric sheet element may include a fourth non-electrode surface portion without electrode material disposed thereon, such that the third non-electrode surface portion is located at another end of the first piezoelectric sheet element in a direction intersecting the tube-axis direction and such that the fourth non-electrode surface portion is located at another end of the second piezoelectric sheet element in a direction intersecting the tube-axis direction.
According to another embodiment of the present invention, there is provided a method of fabricating a piezoelectric conversion element, comprising folding a piezoelectric sheet element having a first electrode formed on a first surface thereof and having a second electrode formed on an opposing second surface thereof such that a fold is produced in the piezoelectric sheet element. The method further comprises rolling the folded piezoelectric sheet element around a shaft, thereby forming a tube member having overlapping layers of the piezoelectric sheet element with a tube-axis direction parallel to the shaft, the fold being oriented parallel to the tube-axis direction. The method further comprises calcinating the tube member at a temperature sufficient to bind the layers of the tube member together, the shaft being incinerated during the calcinating. The first surface of the of the piezoelectric sheet element includes a first non-electrode surface portion without electrode material disposed thereon, the first non-electrode surface portion being located at a first end of the piezoelectric sheet element in a direction intersecting the tube-axis direction. In addition, one of the first and second surfaces may include a second non-electrode surface portion without electrode material disposed thereon. The second non-electrode surface portion may be located on the first surface at a second end of the piezoelectric sheet element, the piezoelectric sheet element being configured such that the fold causes the second surface to confront itself. Alternatively, the second non-electrode surface portion may be located on the second surface at one of the first end and a second end of the piezoelectric sheet element, the piezoelectric sheet element being configured such that the fold causes the second surface to confront itself. The first and second non-electrode surface portions may have a predetermined width approximately equal to a thickness of the piezoelectric sheet element, and the thickness referred to may be the thickness prior to calcinating. In addition, at least one of the first and second surfaces of the piezoelectric sheet element may be provided with a non-electrode surface portion without electrode material disposed thereon in a region encompassing the fold.